1. Field of the Invention
The present invention relates to an inspection apparatus for detecting the presence/absence of defects such as abnormalities in the external shape, surface discoloration, density abnormalities, and the like, with regard to a cylindrical body such as a nuclear fuel pellet (hereinafter referred to as "pellet") and the like.
2. Background Art
Conventionally, a pellet to be loaded in a fuel rod is manufactured by means of forming nuclear fuel powder such as uranium dioxide or the like into a cylindrical green compact and sintering this green compact. With regard to this type of pellet, defects such as cracks and chips sometimes occur at the peripheral surface and/or opposite end faces during the manufacturing process; the pellet with these defects must be removed from the manufacturing process as a defective product. For this reason, a product pellet inspection apparatus is normally provided in the pellet manufacturing process.
An example of this type of pellet inspection apparatus is disclosed in Japanese Patent Application, First Publication, Laid-Open No. Hei 6-66990 by the inventors of the present invention. With regard to this inspection apparatus, a pellet which is ground to predetermined dimensions is transported to a pellet-drying/directional converter, and after drying end faces and the external circumferential surface therein, is inspected by the pellet inspection apparatus, and defective one is removed therefrom. Following inspection for surface defects by means of the aforementioned inspection apparatus, the pellet is further checked by visual inspection by a worker, and only the product free of defects is stored in a tray storage rack.
In the aforementioned pellet inspection apparatus, an image of the circumferential surface of the pellet is picked up by a circumferential surface inspection mechanism, such as a camera or the like, while rotating the pellet. Images of both end faces of the pellet are also picked up by an end face inspection mechanism. The above picked-up image data are then, for example, binary digitized by an image processing mechanism to determine the presence or absence of defects. In other words, in the case when defects such as pits, cracks, chips, dust adhesion, or the like are present on the circumferential surface and/or end faces of the pellet, the quantity of light reflected from these defective portions is less than the quantity of light reflected from surfaces free of defects, and thus it is possible to detect the presence or absence of such defects by means of the aforementioned image data processing.
However, when conducting visual inspection of pellet P using the aforementioned inspection apparatus, if the pellet is formed from a substance exhibiting a low reflectivity, it becomes very difficult to detect the aforementioned defects. For example, when the object to undergo inspection is a Gd pellet (gadolinium-containing uranium dioxide pellet), MOX pellet (uranium dioxide-plutonium dioxide mixed pellet) or the like, the S/N ratio of the pellet reflectivity with respect to the illumination light from the illuminating light source is poor. As a result, even when the surface to be inspected of the pellet is a normal surface free of defects, or alternatively, even when the surface to be inspected is defective, i.e., possessing cracks, chips and the like, a clear difference is not generated in the quantity of light received. Consequently, when attempting to detect the presence or absence of defects based on surface image data taken by a camera from a pellet exhibiting a poor S/N ratio of reflectivity with respect to the illuminating light, the gray level difference between the image data of normal surfaces and that of defective surfaces is small. As a result, misdetection occurs frequently, such that highly precise discrimination of defective pellets is not possible.
In addition, in order to precisely measure the volume and density of the pellet, measurement of the pellet dimensions is also performed. However, conventionally, in order to measure the pellet dimensions, it is necessary to first conduct sampling of the pellet and measure the pellet in an off-line manner. As a result, it is very difficult to conduct these measurements within a short period of time, and it is not possible to measure the entire quantity of pellets. In particular, in the case of a low-density pellet such as an MOX pellet, in addition to the inspection for surface defects, confirmation of the pellet density poses extreme importance; hence, there exists a strong demand for an apparatus which enables 100% inspection of pellets for density abnormalities at a high speed.